Waka Kotahi will soon begin work on a replacement for Onetai Bridge, about halfway between Paeroa and Kōpū on SH26, marking the first state highway bridge built from timber in 50 years.

“It’s the timber elements of the new design that make the project more than just a run-of-the-mill replacement,” said Waka Kotahi regional manager maintenance and operations, Rob Campbell.

“Road bridges in Aotearoa New Zealand are generally built with concrete or steel. Onetai will be the first to mark a return to more sustainable design, with timber beams and deck.

“We’re hoping to make this the new norm, so while it might seem like a minor project, it marks a ground-breaking shift in design for bridge replacements on state highways.”

It all comes down to the magic of layering – using an engineered timber product called glulam, which is made from layers (laminates) of wood glued together. As Campbell explains, glulam has twice the strength per kilogram compared with steel, and is specially designed to be hard-wearing and durable.

“The new Onetai Bridge is designed to last at least 100 years, linking the greater Waikato region with the Coromandel Peninsula, and providing a small but vital part of our state highway network,” he said.

“The replacement will add to the resilience and safety of this route, as the new design will be stronger and wider than the old bridge.”

Glulam is being used increasingly around the world for bridges, both on highways and on local roads.

“The first and most obvious benefit is that glulam’s main component, pine wood, is grown right here in Aotearoa and therefore does not need to be mined or go through the high-energy manufacturing processes needed to make steel and concrete,” said Campbell.

The new Onetai Bridge will be the same length as the old one, but will be 3m wider, and boasts extra safety features. These include a new type of side barrier that can be fitted right to the outside edge of the bridge, and therefore gives drivers more lane space, helping reduce the risk of head-on crashes.

The bridge will have simple concrete foundations, and new scour protection both upstream and downstream. Scour, which happens when water flows change or rivers flood, is a leading cause of bridge failure. With the climate changing rapidly, there is a renewed urgency in using scour protection to maintain the safety of bridges.

Waka Kotahi undertook a business case in 2020 for the replacement of the existing two-lane bridge, which, while still safe to use, is nearing the end of its life. Construction funding was approved under the current 2021–24 National Land Transport Programme, and the project has moved into the detailed design stage, with construction planned to begin this October.

In the meantime, Onetai Steam Bridge remains safe to use.